The inhibitory effect and mechanism of the apple
dihydrochalcone,
phloretin, on
breast cancer cell growth were evaluated in in vitro conditions simulating complete nutrition and
glucose-restriction, respectively. In two
breast cancer cell lines with different histological backgrounds,
phloretin consistently exhibited much stronger activity against cell growth in
glucose-limiting than in full media.
RNA-seq analysis showed that key autophagy-related genes were downregulated upon
phloretin treatment in both
estrogen-receptor-positive MCF7 and triple-negative MDA-MB-231 cells. Immunoblotting verified significantly decreased expression of LC3B-II by
phloretin in low-
glucose and
glucose-free media, but not in full medium. Together with the use of two pharmacological autophagy inhibitors,
chloroquine and
3-methyladenine, and confocal microscopy of
breast cancer cell lines transfected with GFP-LC3B,
phloretin demonstrated a strong capability to suppress autophagic flux, which was likely mediated through downregulation of mTOR/ULK1 signaling, whereas the expression of canonical autophagy regulators ATG5 and ATG7 was not significantly affected.
Phloretin also reversed
tamoxifen- and
doxorubicin-induced cytoprotective autophagy in the
breast cancer cell lines, and this was manifested in its synergistic growth inhibitory effect with these chemotherapeutic agents. Furthermore, it was able to restore or enhance the chemosensitivity of a
tamoxifen-resistant cell line. Taken together, our study has, for the first time, revealed that
phloretin could effectively suppress
glucose-
starvation- and chemotherapeutic-induced cytoprotective autophagy in
breast cancer cell lines likely through downregulation of mTOR/ULK1 signaling.